Are you pasting a link to MSN in a comment, and then you get that error? My guess would be that when you paste the link, FB tries to connect with MSN to pull some data to create a rich thumbnail, and that is not working for some reason. (Although I've never see that issues myself with sites I have linked to.)

Do you use the Facebook Container extension or any other add-ons that might be blocking cross-site interactions between Facebook and MSN?

I am not using the Facebook Container extension, any time I'm reading a news item from say Politico, NBC News, CNN etc, there are four Icon at the top each news item to share on Facebook, Twitter, Phone or email, when I attempt to share with Facebook, I get this error message, the other 3 links to share work but Facebook does not.

Have you tested this sharing button in another browser to see whether it is a "Firefox only" problem?

So for example on this story:

https://www.politico.com/news/2020/10/19/canada-us-border-coronavirus-nonessential-travel-430145

(Note: including a non-Mozilla URL in a reply usually delays the appearance of your post while it passes through link moderation.)

Each of the sharing buttons runs through AddThis, for example:

http://api.addthis.com/oexchange/0.8/forward/facebook/offer?pco=tbx32nj-1.0&url=https://politi.co/35h23Yl&pubid=politico.com

When I click that, I get a page from Facebook that says

"Log in to use your Facebook account with Shared via AddThis."

Since I don't want that service to access my Facebook, I'm going to pause here.

I suspect the problem occurs when you have already granted permission to an AddThis app. Maybe try removing it and adding it again to see whether that helps?

To edit or remove an app permission in Facebook, see the steps in this article:

https://www.facebook.com/help/218345114850283/